Trigate and high-kstack up vs. planar
Austin, Texas -- Intel Corp. researchers will present a vertical triple-gate structure combined with a high-k oxide and metal electrode gate stack at the 2006 Symposium on VLSI Technology, which kicks off tomorrow in Honolulu. The company has also devised strain techniques unique to the vertical structure.
The combination yields a transistor that Intel claims outperforms the planar transistors in use today. Mike Mayberry, Intel's director of components research, said the device's power consumption, both active and standby, is 35 percent better than that of the current 65-nanometer transistors.
While Mayberry called the trigate transistor "a serious candidate" for Intel beyond the 45-nm node, skeptics argue that vertical transistors present manufacturing problems that worsen as the distance between transistors, or pitch, continues to narrow.
Vertical transistors, also called FinFETs, create a silicon channel that resembles a tall fin. Interest in FinFETs is growing as the channel length for planar transistors shrinks to less than 20 nm for the 32-nm technology generation, posing problematic short-channel effects.
And interest is not confined to logic devices. At the VLSI symposium this week, researchers from Samsung Electronics Co. will present no fewer than five papers on FinFET structures aimed primarily at NAND flash memories.
As planar CMOS scales, Mayberry said, it becomes more difficult to turn the devices off completely. With the high-k version of the trigate transistor, Intel has demonstrated a subthreshold slope--a metric of how fast the device can be turned off--that is about 45 percent better than that of Intel's planar 65-nm transistors. (The company is keeping its 45-nm transistor performance under wraps.) The trigate is also said to be superior in another measure of performance: the time required to full saturation, or going from a fully off state to fully on.
"Multigate structures allow you to turn off the device more cleanly," Mayberry said. "The silicon is closer to the gate in vertical structures, so they can be turned off more efficiently. And we can more quickly turn on the devices--they switch more efficiently.
"The use of high-k reduces the gate leakage and allows you to strongly couple the gate with the channel. The strain techniques improve the mobility. We've combined all of this into one device, and it's work that we're proud of."
Freescale Semiconductor Inc. also is pursuing a modified FinFET, combining a planar channel with a tall fin. "At a high level, these multigate devices are the best at controlling the transistor function," said Bruce White, an advanced CMOS technology manager at Freescale. "They turn on well, and they turn off well. The challenge is that all of the infrastructure needs to be there to manufacture them efficiently."
Ghavam Shahidi, director of silicon technology at IBM's T.J. Watson Research Center at Yorktown Heights, N.Y., said he remains skeptical that vertical transistors will ever be brought into manufacturing.
IBM researchers will present research at the VLSI symposium that examines the challenges of creating 32-nm vertical devices at a 120-nm transistor pitch. "As we go down the nodes, spacings are getting closer together," Shahidi said. "For these high-aspect-ratio, three-dimensional devices, the [structures] are getting harder to fit together." Between the fins, companies must create spacers, add the silicide, place dopant implants and build the contacts--tasks that are difficult enough to achieve for planar devices and that become even more burdensome for vertical structures.
"FinFETs are not our front option" for the nodes beyond 45 nm, Shahidi said, partly because of the manufacturing difficulties. "With all these devices, the silicon sticks up, and the gate wraps around it. We can easily make one isolated FinFET, but when we have to make billions of them, at these close pitches, all the trigates and FinFETs get harder and harder. Intel is doing excellent research, they are a great company . . . but I'm surprised that Intel is pushing trigate."